Genomic structure of the human complement protein C8 gamma: homology to the lipocalin gene family.

Human C8 is one of five complement components (C5b, C6, C7, C8, C9) that interact to form the cytolytic C5b-9 complex on target cells. It contains three subunits (C8 alpha, C8 beta, C8 gamma) which are encoded in separate genes. In relation to other proteins of the complement system, C8 gamma is unusual in that it is not structurally related to any other component nor does it have an obvious function. Based on weak but significant sequence similarity, it is proposed to be a member of the lipocalin family of widely distributed proteins that bind and transport small hydrophobic ligands. In this study, the human C8 gamma gene has been characterized and found to contain seven exons spanning approximately 1.8 kb. S1 nuclease and anchored PCR were used to identify the transcription initiation site. This site is preceded by putative regulatory elements that include two SP1 binding sites, several glucocorticoid response elements, and two SV40 enhancer core consensus sequences. A comparison to genes of other lipocalins reveals a remarkably close correlation in exon number, lengths, and phases. A close correspondence in exon boundaries is also observed and suggests that C8 gamma contains the same discrete structural elements that define the characteristic beta-barrel shape of the lipocalins. These results establish that C8 gamma is indeed ancestrally related to the lipocalin family and strengthens the likelihood that its role in the complement system is to bind an as yet unidentified ligand.